Factors which determine the total bacterial content of a water supply include the following:
It is not the total bacterial content, however, which is important in assessing the suitability of a water supply for human consumption. What matters is the possibility that it contains organisms capable of causing disease in those who drink the water. Important among these organisms are the bacilli of typhoid, paratyphoid, dysentery and cholera; some pathogenic viruses can also be water-borne—notably those of poliomyelitis and hepatitis A.
Presence of such organisms results from contamination with human excreta, or in some cases with animal or bird droppings. There are a number of obvious precautions which can be taken to reduce the risk of such contamination—e.g. drawing supplies only from relatively uninhabited catchment areas, seeing that one community is not discharging its untreated sewage into a river upstream from the point at which another is drawing off its supply, or, in places where there is no main water supply or main drainage, seeing that wells are so situated and protected that there cannot be any leakage into them from pit-latrines and the like.
There are few occasions, however, when it is safe to assume that a water supply has not been contaminated—though a dangerous level of contamination is highly improbable in the case of water taken from a fast-running stream in a hill or mountain area above the level of human habitation. With this exception it is virtually always unwise to drink unboiled water derived from a source that has not been subjected to thorough and repeated testing, as described below, or to filtration and chlorination with adequate bacteriological control. Although most non-chlorinated water supplies contain Gram-negative bacilli, including enterobacteria such as Klebsiella aerogenes, the presence of 'faecal coliforms' such as Escherichia coli indicates contamination with human or animal faeces, and their presence in more than very small numbers indicates that the water is not safe for human consumption. Streptococcus faecalis and Clostridium welchii have a similar significance, though the latter, being a sporing organism, survives longer than other faecal organisms, and its presence without them suggests that contamination was not recent.
There is seldom any point in examining a water supply directly for the presence of pathogens, for if contamination has occurred other faecal organisms are likely to be far more numerous and easier to detect, and the presence of these organisms in a supply that does not at present contain detectable pathogens implies that it may well do so on other occasions.
Since contamination may be intermittent, a single satisfactory bacteriological examination does not guarantee the safety of a water supply. Regular testing of any supply to be used for drinking is essential, and in the case of a non-chlorinated supply any deviation from the pattern of results obtained over the course of previous years must be taken as indicating that there has been a change in the source of the supply and that it must be regarded with suspicion. Almost all piped drinking water in Britain is chlorinated. The presence of any coliform bacilli at all in a too ml sample of chlorinated water collected at its point of entry into a public supply indicates that the chlorination procedure was defective.
BACTERIOLOGICAL EXAMINATION OF WATER
As we have indicated, the most important part of the bacteriological examination of water is the detection and enumeration of coliform bacilli, and in particular of those that are of faecal origin. This can be done by adding portions of the water sample—e.g. one of 5o ml, 5 of to ml and 5 of I ml—to bottles or tubes containing a suitable liquid lactose containing medium and incubating overnight at 37 C.
Production of acid and gas in a bottle or tube indicates the presence of 'presumptive coliforms'. Because organisms may be irregularly distributed in the water, it may well happen that, for example, growth of `coliforms' occurs from some of the 1 ml samples but fails to occur from some of the 10 ml samples.
However, by comparing the results obtained with McCrady's probability table worked out for this purpose, it is possible to arrive at a figure for the most likely number of `coliforms' per 100 ml of the water. Not all bacteria that produce acid and gas under the conditions of the presumptive coliform count are in fact faecal organisms. If any such bacteria are detected, they are further tested for ability to grow and to produce gas in the same or a modified fluid medium incubated at 44 C.
With rare exceptions, the ability to do this is confined to true 'faecal coliforms'. The finding of any such indicates that the water is not fit for human consumption. An alternative procedure for the bacteriological sampling of water is to pass a known volume of water under pressure through a special porous cellulose acetate membrane which holds back all bacteria. The membrane can then be placed in a Petri dish on top of a pad of filter paper or other suitable absorbent material which is saturated with a fluid culture medium.
On incubation, colonies form on the surface of the membrane. By using appropriate media and incubation temperatures it is possible to carry out total counts, counts of lactose-fermenting organisms that will grow on bile-salt lactose medium and counts of those which will do so at 44 C. However, gas production cannot be detected and the results are therefore not strictly parallel with those of the tests in liquid media.
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